Causes

The production of sperm is a complex process and requires normal functioning of the testicles (testes) as well as the hypothalamus and pituitary glands — organs in your brain that produce hormones that trigger sperm production. Once sperm are produced in the testicles, delicate tubes transport them until they mix with semen and are ejaculated out of the penis. Problems with any of these systems can affect sperm production. Also, there may be problems of abnormal sperm shape (morphology), movement (motility) or function. Often the cause of low sperm count isn't ever identified.

Medical causes

Low sperm count can be caused by a number of health issues and medical treatments. Some of these include:

Varicocele. A varicocele (VAR-ih-koe-seel) is a swelling of the veins that drain the testicle. It's a common cause of male infertility. This may prevent normal cooling of the testicle, leading to reduced sperm count and fewer moving sperm. The treatment of a varicocele often improves sperm quality and overall fertility.

Infection. Some infections can interfere with sperm production and sperm health or can cause scarring that blocks the passage of sperm. These include some sexually transmitted infections, such as chlamydia and gonorrhea; inflammation of the prostate (prostatitis); inflamed testicles; and other infections of the urinary tract or reproductive organs.

Ejaculation problems. Retrograde ejaculation occurs when semen enters the bladder during orgasm instead of emerging out of the tip of the penis. Various health conditions can cause retrograde ejaculation, including diabetes, spinal injuries, and surgery of the bladder, prostate or urethra. Certain medications also may result in retrograde ejaculation, such as blood pressure medications known as alpha blockers. Some men with spinal cord injuries or certain diseases can't ejaculate semen at all, though they still can produce sperm.

Antibodies that attack sperm. Anti-sperm antibodies are immune system cells that mistakenly identify sperm as harmful invaders and attempt to destroy them.

Tumors. Cancers and nonmalignant tumors can affect the male reproductive organs directly, or can affect the glands that release hormones related to reproduction (such as the pituitary gland). Surgery, radiation or chemotherapy to treat tumors can also affect male fertility.

Undescended testicles. During fetal development one or both testicles sometimes fail to descend from the abdomen into the sac that normally contains the testicles (scrotum). Decreased fertility is more likely in men with this condition.

Hormone imbalances. The hypothalamus, pituitary and testicles produce hormones that are necessary to create sperm. Alterations in these hormones, as well as from other systems such as the thyroid and adrenal, may impair sperm production.

Sperm duct defects. The tubes that carry sperm can be damaged by illness or injury. Some men are born with a blockage in the part of the testicle that stores sperm (epididymis) or a blockage of one of the tubes that carry sperm out of the testicles (vas deferens). Men with cystic fibrosis and some other inherited conditions may be born without sperm ducts altogether.

Chromosome defects. Inherited disorders such as Klinefelter's syndrome — in which a male is born with two X chromosomes and one Y chromosome instead of one X and one Y — cause abnormal development of the male reproductive organs. Other genetic syndromes associated with infertility include cystic fibrosis, Kallmann's syndrome and Kartagener syndrome.

Heavy metal exposure. Exposure to lead or other heavy metals also may cause infertility.

Radiation or X-rays. Exposure to radiation can reduce sperm production. It can take several years for sperm production to return to normal. With high doses of radiation, sperm production can be permanently reduced.

Overheating the testicles. Frequent use of saunas or hot tubs may temporarily lower your sperm count. Sitting for long periods, wearing tight clothing or using a laptop on your lap for long periods of time also may increase the temperature in your scrotum and reduce sperm production. The type of underwear you choose to wear is unlikely to significantly impact your sperm count.

Prolonged bicycling. Prolonged bicycling is another possible cause of reduced fertility due to overheating the testicles.

Health, lifestyle and other causes

Other causes of low sperm count include:

Illegal drug use. Anabolic steroids taken to stimulate muscle strength and growth can cause the testicles to shrink and sperm production to decrease. Use of cocaine or marijuana may temporarily reduce the number and quality of your sperm as well.

Sperm testing issues. Lower than normal sperm counts can result from testing a sperm sample that was taken too soon after your last ejaculation; was taken too soon after an illness or stressful event; or didn't contain all of the semen you ejaculated because some was spilled during collection. For this reason, results are generally based on several samples taken over a period of time.

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